Liquid crystal display devices are commonly constituted of providing a polarizing plate on both sides of a liquid crystal cell. The polarizing plate transmits light of a plane of polarization in a fixed direction. Consequently, in liquid crystal display units, the polarizing plate plays a critical role in which variation of orientation of liquid crystals due to an electric field is visualized. Accordingly, the performance of liquid crystal display devices significantly varies depending on the performance of polarizing plate.
Production methods of optical films are divided mainly into a solution casting film forming method and a melt casting film forming method. The former is a method in which polymers are dissolving in solvents, the resulting solution is cast, the solvents are evaporated, and if desired, stretching is performed to prepare film. This method has been widely employed, since it results in excellent uniformity of film thickness. However, it has also resulted in problems such as an increase in size of facilities. The latter is a method in which polymers are heat-melted, cast onto a support, cooled, and solidified, and if desired, stretched to form film. Since it is not necessary to dry solvents, one advantage is that it is possible to downsize facilities. However, problems of uniformity of film thickness result.
In recent years, it is common to employ retardation correcting film in liquid crystal display units. Along with larger image screens and higher definition, quality demanded for retardation films becomes more sever. Specifically, in the retardation film exhibiting larger in-plane retardation, the demand in the slow axis (being the orientation axis) direction (being an orientation angle) is more severe, and it is desired that the accuracy is commonly at most angle of 1° over the entire area within the film but is preferably in the range of about ±0.3 to about 0.5°.
Commonly employed as such a retardation film is one which is prepared by uniaxially stretching a polycarbonate based resinous film, exhibiting a relatively large intrinsic double refractive index, in the longitudinal direction (being the film running direction during production). However it has not been possible to obtain a positive wavelength dispersion characteristic by employing only a single polycarbonate based retardation film.
Further, the slow axis of the above retardation film is in the longitudinal direction, which equals the stretching direction. In cases in which the retardation film for VA mode liquid crystals is adhered to a polarizing film, it is necessary that the slow axis is directed toward the transverse direction (being perpendicular in the film plane with respect to the uniaxial stretching direction of a polarizing film). However, it is not possible to allow a retardation film exhibiting the slow axis in the longitudinal direction to adhere to the polarizing film in the form of a long roll, and it is necessary to adhere to each other matching the direction while cutting it into sheets. Consequently, problems have occurred, whereby productivity is markedly degraded.
On the other hand, in view of enhancement of productivity, preferred is a film whose orientation angle is in the transverse direction (being the TD direction) of a long film, because it is possible to perform production in the roll form during the polarizing plate adhesion process. The above film whose orientation angle is in the TD direction is frequently produced employing resins such as polycarbonates or cellulose ester based resins, which are subjected to molecular orientation (exhibiting positive birefringence) in the stretching direction while employing a lateral stretching apparatus.
Further, it is required that the slow direction of retardation film for IPS mode liquid crystals matches the longitudinal direction of a polarizing film. In this case, when stretching is performed in the above TD direction, it is not possible to achieve adhesion in the form of a roll, whereby productivity is degraded. Consequently, film is produced by stretching in the MD direction or in the TD direction employing materials such as polystyrene based resins or acrylic resins which are subjected to molecular orientation (exhibiting negative birefringence) perpendicular to the stretching direction. The MD direction designates a film conveying direction.
In the lateral stretching process employing a tenter, film is heated to a temperature optimal for stretching and stretched in the TD direction. It is well known that bowing phenomena occur in which a straight line (being a stretching line) drawn in the TD direction of the film prior to stretching is curved in the form of an arc after stretching.
When such bowing phenomena occur, problems result in which the orientation axis of the retardation film is aligned in the tangential direction of the extension of the arc, whereby the resulting orientation angle is not uniformly directed in the TD direction. Since the bowing phenomena vary depending on stretching conditions, various technologies to minimize the bowing phenomena have been disclosed.
Incidentally, in cases in which the bowing phenomena are eliminated by working out stretching conditions (wherein the stretching line is in a straight line), the film in the tenter becomes is softened via heating, whereby the orientation angle results in distribution in the transverse direction due to a mechanical asymmetric property of the tenter. Further, when temperature distribution results across the width in the tenter, the softness of the film differs across the width to result in non-uniform stretching, whereby the orientation angle results in distribution.
Further, other than the tenter stretching apparatus, are many other factors which result in non-uniform orientation angle in the transverse direction.
During production of optical film, close attention is paid so that non-uniformity of the film thickness in the transverse direction is minimized through the conveying line, the heating/drying equipment, and casting. However, right and left mechanical uniformity in the production line is degraded over a period of time due to thermal distortion repeatedly applied to the production facilities and abrasion of sliding sections, whereby the resulting orientation angle varies over an elapse of time.
Further, in cases in which an optical film is produced in such a manner that a film prepared employing a solution casting film forming method is subjected to in-line stretching the conveyed film is softened due to incorporation of solvents and is greatly influenced by the right and left non-uniformity of the conveying line, whereby the orientation angle of the film tends to result in distribution in the transverse direction. The in-line stretching designates a method in which the film formed is stretched continuously without interruption.
Still further, film which has been peeled from the support results in optical characteristic distribution in the transverse direction due to non-uniform thickness and non-uniform drying in the transverse direction. Such distribution in the transverse direction is pronounced when the casting rate is increased to enhance productivity.
During production of a high precision optical film, especially a retardation film, it is essential that the distribution in the transverse direction of the above orientation angle is maintained within the desired accuracy range.
In the production method of film employing a transverse stretching apparatus, a method is not substantially available which precisely controls the orientation angle in the longitudinal or transverse direction.
Conventionally, a film at an orientation angle of 0°/90° has been produced by arranging the conveying line and the stretching apparatus to be as uniform as possible on the right and the left with respect to the machine center. However, mechanical accuracy tends to vary over an elapse of time, whereby precise control is required.
In the production method of optical films employing a lateral stretching apparatus, disclosed as a method for controlling the orientation angle are many technologies which result in an oblique orientation angle with respect to the MD direction of the film. For example, in Patent Documents 1 and 2 below, film production methods are proposed which employ lateral direction film stretching apparatuses in which the running rate and distance of the right and left clips differ.
Patent Document 1 and the method described therein disclose a technique in which by declining the orientation axis by 45° in the longitudinal direction of the film, lateral and longitudinal film strength is allowed to be uniform in the lateral direction/longitudinal direction.
Further, similar optical film production methods are disclosed in Patent Documents 3-5 below. These also disclose techniques to decline the orientation axis by 10-80° with respect to the longitudinal direction.
(Patent Document 1) Japanese Patent Publication for Public Inspection (hereinafter referred to as JP-A). No. 50-83482
(Patent Document 2) JP-A No. 2-113920
(Patent Document 3) JP-A No. 3-124426
(Patent Document 5) JP-A No. 4-164626